This is the ID used within the Vagrantfile. It is only used for your internal identification.
The hostname the machine should have. This answer lists all valid characters.
The Key Pair name from here.

The EC2 server will not be accessible without your

Choose the one closest to you, or your target audience!

The AMI must be available to the region and size

You must know what username your chosen AMI requires for initial boot up!
Assign security groups to this instance. Get them here.
The subnet to boot the instance into, for VPC. Get them here.
Assign an elastic IP address to this instance. Get them here.